Hey, I have a 89' Prelude 2.0Si 5-Speed and the speedometer went on me about a week ago. I'm looking online for a replacement speed sensor, but I can't find one anywhere. Any ideas? Already checked autozone, ebay, and advance.
Somebody mentioned "Wells" in this thread, [ Gas gauge dead & speedo cutting out... ] but I can't find them either.

Welcome to the forum!

Just to be clear, exactly which part are you looking for? 88/89 Preludes have a cable driven VSS, and the actual VSS is located in the back of the gauge cluster. Are you looking for the piece that is mounted to the transmission that the cable connects to, or are you looking for the VSS? The 90/91 Preludes do not use a speedometer cable, and the VSS is mounted to the transmission. Your car is not like the OP's car in the thread you attached in your post.

Also, are you sure the problem is the VSS? Are you getting a check engine light for a VSS (code 17)? Does the odometer still work? If not, then you are more likely to have a speedometer cable issue. It would behoove you to do a little troubleshooting before you start replacing parts.

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1991 Prelude Si ALB 4WS
2017 Ram Rebel

I'm getting code 17 and I haven't driven it enough to see if the odometer is working, but the tripometer is working. It's an 89' so I would suppose it's the VSS up by the instrument cluster, but I still can't find the part online.

Unfortunately you won't be able to find one online. You'll have to go with used if you want to replace that part. Junkyards, ebay, craigslist, car-part.com, part-outs, etc. I had a spare for a while but I sold the gauge cluster that housed it.

If the trip meter is working then the cable is working. You might want to follow the troubleshooting guide here...
http://media.honda.co.uk/car/owner/m...f100/6-170.pdf

In looking at that troubleshooting guide, it seems just as likely to be a problem with the ECU as with the VSS. It would suck for you to go through the hassle of finding a VSS, only to find that it doesn't fix your problem.

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1991 Prelude Si ALB 4WS
2017 Ram Rebel

Is this the part I'm looking for (provided it's not the ECU)?
GENUINE OEM 1988-1989 88 89 Honda Prelude VSS Speed Sensor 5-SPEED MANUAL CABLE | eBay

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1989 Honda Prelude 2.0Si 5 Speed
1992 Acura Legend LS 5 Speed

No, that's the mechanical part that mounts to the transmission. If your cable is functioning properly you can eliminate that part as a cause for your issue.

If your problem really is the VSS, you probably won't find it separate from the gauge cluster since it's actually mounted inside the cluster.

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1991 Prelude Si ALB 4WS
2017 Ram Rebel